This invention relates to data communication systems, and in particular, to an equalizer for a digital transmission system, which compensates for distortion in a data signal at any data rate over a transmission line.
Known methods for equalization of data communication network signals can be divided into two general categories: line build-out schemes and direct compensation of the transmission line characteristics (i.e. direct equalization). Automatic line build-out techniques function by degrading the received signal to a known operating level representative of the maximum anticipated transmission line distance. A fixed equalizer circuit is used to restore the data signal to levels where the data can be recovered. Direct equalization techniques, on the other hand, restore the received data signal to a desired level without additional degradation. A direct equalization circuit provides restoration commensurate with the level of degradation in the data signal.
Known direct equalization methods often utilize some form of closed loop control, which is based on signal amplitude. Amplitude information is indicative of the cable characteristics because amplitude is closely controlled at the sending end of the transmission line. A major drawback to the known techniques is the inability to provide the best degree of compensation at the various operating data rates and line lengths. This problem is particularly severe when operating at very high data rates; for example, 56 kilobits/second (kb/s), and long transmission lines, for example, greater than 4-5 kilometers.